Various Important Laboratory Tests And Miscellaneous Question And Answers

Various Important Laboratory Tests

 

Question 1. Write a short note on
1. Widal Test
2. Elisa Test
Or
Write a short note on ELISA.
Or
Write a short note on the WIDAL test
Answer:

1. Widal Test:

This test is the most widely used.

Principle: This is an agglutination test that detects antibodies that are produced against S. typhi, S. paratyphi A, and B.

Antigens used:

• In this test, four antigens are used
  • H and O of S. typhi
  • H of S. paratyphi A
  • H of S. paratyphi B
• Usually, antigens of paratyphoid O are not used in this test as they cross-react with typhoid O antigens by sharing factor 12.

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Procedure:

• The widal rack has four rows of test tubes.
• Add an equal volume of serial dilution of serum to all test tubes.
• Add antigen to the respective rows.
• Incubate the rack in a water bath and set it at 37 °C overnight.
• Control tubes consist of normal saline and antigens are used to check autoagglutination.

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Interpretation:

• H agglutination: Appear as loose cotton woolly clumps.
• O agglutination: It appears as a granular disc-like pattern at the bottom of the test tube.
• Significant titers depend on the titer found in local healthy persons.
• Titers of 1:200 for H and 1:100 for O are considered to be significant.
• The following points shall be considered while interpreting the results of the Widal test:
• A single positive test result cannot diagnose enteric fever and a negative test result should not rule out the presence of enteric fever.
• Demonstrating a rising titer in the test mode in the first and the third week is of high significance than a single test.
• If the collection of the first sample is late in the disease, the rise is not demonstrated.
• Antibodies appear during the 7th to 10th day of enteric fever so that negative test results may be obtained from samples that are collected directly.
• Titre increases steadily till 3rd or 4th week of enteric fever.

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• Serums taken from the individuals who are immunized by TAB vaccine demonstrate high titer antibodies against all antigens but in patients having typhoid fever rise is only against one serotype.
• The persistence of H agglutinins is for many months after vaccination but O agglutinins disappear soon.
• The rise in O is indicative of a recent infection.
• For a demonstration of stereotype on an infecting organism, H agglutinin is more reliable as compared to O because serotypes have shared O antigen.
• Persons having past history of typhoid fever or immunization can develop an anamnestic response during unrelated fever.
• This difference is demonstrated by repeating the test after a week.
• The anamnestic response demonstrates transient rise, while in typhoid fever the rise can sustain.
• Poor agglutinin response is shown by the patient treated with chloramphenicol.
• In non-specific conditions such as rheumatoid arthritis, multiple myeloma, etc. due to fibrial antigen false positive results can be produced.

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2. ELISA Test

• The full form of ELISA is Enzyme-linked Immunosorbent Assay
• The antigen obtained from HIV grown in continuous T lymphocyte cell line or by recombinant technology is coated on microtiter wells.
• The test serum is added and if the antibody is present it binds
  the antigen.
• After washing away unknown serum antihuman immunoglobulin linked to the suitable enzyme is added, followed by the color forming substrate.
• If the test serum contains an anti-HIV antibody, a visible or photometrically detachable color is formed which can be read visually or by special ELISA readers.

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Miscellaneous

Question 2. Classify pus-forming organism.
Answer:

The following are the pus-forming organisms:

• Gram-negative bacilli:
  • Salmonella typhi
  • S. paratyphi A, B, C
  • Brucella spp.
  • Haemophilus influenza
  • E. coli
  • Klebsiella pneumoniae
  • Proteus spp.
  • Enterobacter spp.
  • Bacteroides spp.
  • Pseudomonas spp.

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• Gram-positive cocci:
  • S. aureus
  • Staphylococcus epidermidis
  • S. pyogenes
  • Streptococcus pneumoniae.
• Gram-positive bacilli:
  • Listeria monocytogenes.
• Gram-negative cocci:
  • Neisseria meningitidis.